Flexible mandrels have been in existence for many years. Initially they were solid cylindrical extrusions of rubber or plastic upon which the various layers of polymers and reinforcement were built to form tubular products. These suffered from lack of dimensional stability in both the radial and the longitudinal directions as well as the inability to repair or connect broken sections. The next modification was to introduce a reinforcing cable about which the rubber or plastic cylinder was extruded to form a mandrel. While this provided dimensional stability and repairability, the service life was shortened as a result of using water pressure to blow the flexible mandrel out of the tubular product. It is believed that the water entered the core of the cable reinforcement thereby attacking the wire and destroying the rubber or plastic to wire adhesion which in turn caused a loss of dimensional stability.
In order to overcome this wicking action of water entering through the core of a wire cable, the next modification was to use a non-metallic cable made of a somewhat compressible thermoplastic material, e.g., a polyamide. This solid or tightly stranded material when used as the core for the extruded rubber or plastic cylinder provided dimensional stability and prevented the wicking action of the water entering the core. However, the broken or cut lengths could not be repaired.